1. Field of the Invention
This invention relates to a method for functionalizing block copolymers. More particularly, this invention relates to a process for incorporating carboxyl and carboxyl derivative functionality into a block copolymer.
2. Prior Art
Polymers modified so as to contain a carboxylic acid group or a derivative thereof are, of course, well known in the prior art. For example, polymers containing ethylenic unsaturation can be modified by reacting the polymer with an unsaturated carboxylic acid or a derivative thereof thermally through the so called ENE reaction as taught, for example, in U.S. Pat. Nos. 4,292,414; 4,308,353 and 4,427,828. When the ethylenic unsaturated polymer is modified in this manner, the unsaturated carboxylic acid or a derivative thereof is, in effect, grafted onto the polymer chain at a carbon atom allylic to an ethylenic unsaturation contained in the polymer. Ethylenically unsaturated polymers may also be modified through free radical addition of an unsaturated carboxylic acid across an ethylenic double bond contained in the polymer. Such free radical addition may be accomplished in the presence of a free radical initiator, generally at temperatures within the range from about 160.degree. to about 230.degree. C. Saturated polymers, i.e., polymers which are free or at least substantially free of ethylenic unsaturation, can also be modified by grafting an ethylenically unsaturated acid or a derivative thereof onto the polymer in the presence of a free radical initiator as taught, for example, in U.S. Pat. Nos. 4,578,429; 4,632,769 and 4,678,173 and European Patent Application No. 0171167. When saturated polymers containing hydrogenated conjugated diolefin monomer units are modified, however, the acid or acid derivative is grafted to the polymer at a secondary or tertiary carbon position as taught in U.S. Pat. No. 4,578,429 or at the site of a chain scission as taught in U.S. Pat. No. 4,670,173.
Polymers containing a grafted carboxylic acid compound or a derivative thereof are, of course, known to be useful as impact modifiers in both thermosetting and thermoplastic resin molding compositions such as taught, for example, in U.S. Pat. Nos. 4,329,438 and 4,628,072. Such polymers may also be further modified so as to produce viscosity index improvers having dispersant characteristics as taught, for example, in U.S. Pat. Nos. 4,033,888; 4,077,893; 4,141,847 and 4,670,173. Such modified polymers also exhibit improved adhesive properties and are useful in a rather broad range of adhesive, sealant, coating and related uses.
As indicated in at least certain of the aforementioned U.S. Patents, polymers which are modified so as to incorporate a carboxylic acid compound or a derivative thereof may be prepared using solution techniques. These techniques, however, generally require extended holding times and unless relatively high temperatures are employed the amount of the carboxylic acid compound or its derivative incorporated in the polymer is generally low. Polymers containing a carboxylic acid compound or a derivative thereof may also be prepared by contacting the polymer with an unsaturated carboxylic acid or a derivative thereof in an extruder, generally in the presence of a free radical initiator, as taught, for example, in U.S. Pat. Nos. 4,578,429; 4,632,769 and 4,670,173 and European Patent Application No. 0171167. This technique, will of course, reduce the holding time required to effect the reaction and can, generally, increase the amount of carboxylic acid or derivative thereof incorporated into the polymer. As indicated in U.S. Pat. No. 4,670,173, however, this method of preparing the modified polymer results in chained scission of the polymer thereby reducing the polymers molecular weight. Moreover, when the chain scission occurs in the central block of a triblock copolymer such scission can significantly impair the polymer's performance. It is also known that the contacting of a polymer and a free radical initiator in an extruder can result in significant coupling or crosslinking of the polymer. It has now been discovered that degradation (scissioning) of the polymer is significantly greater when the polymer to be modified in an extruder is a radial polymer as opposed to a linear polymer. Degradation (scissioning) is, of course, a significant debit when the modified polymer is ultimately to be used as a viscosity index improver since the resulting reduced molecular weight will significantly reduce thickening efficiency. Also, coupling or crosslinking could lead to the production of polymeric materials which are not soluble in the oil composition which might contain the viscosity index improver. An improved process for modifying hydrocarbon copolymers, and particularly copolymers which are ultimately to be used as viscosity index improvers, which will result in less degradation and/or coupling or crosslinking while at the same time requiring reduced holding times, is, therefore, believed to be readily apparent.